Hydrogen Sulfide Minimization
This invention relates generally to catalysts used to reduce the amount of hydrocarbons, carbon monoxide, and nitrogen oxides in automotive exhaust gas, and in particular to promoted delta-alumina supported catalysts which have high durability although they do not contain cerium oxide and which, in addition, minimize the amount of hydrogen sulfide in the exhaust gas without the use of nickel oxide.
In the past few years, automotive catalyst technology has improved such that the present-day catalysts have much higher activity and durability than in the past. Part of this improvement has been achieved by increasing the content of oxygen storage components such as cerium oxide which serve to "not only promot[e] the oxidation activity but also widen[] the air-fuel ratio window where all three major pollutants, HC, CO, and NO can be removed." (J. Catal., 86, 254 [1984]). Unfortunately, these oxygen storage components (e.g., cerium oxide) also have a detrimental effect: the components store sulfur on the catalyst and release virtually all of the stored sulfur over a very short period of time in the form of the foul-smelling compound hydrogen sulfide.
Hydrogen sulfide is formed over a catalyst in a number of different ways. The noble metal in an automotive catalyst may catalyze the conversion of sulfur oxides present in the exhaust stream to hydrogen sulfide. Also, sulfur can be stored on the catalyst support (e.g., alumina) as the sulfate under oxidizing conditions and later converted to hydrogen sulfide under reducing conditions (J. Catal., 66, 231 [1981]). Finally, sulfur may be stored on the oxygen storage component of the support under oxidizing conditions and later converted to hydrogen sulfide under reducing conditions. It is the larger release of hydrogen sulfide which follows the storage of sulfur on the oxygen storage component which has motivated researchers to try to minimize the hydrogen sulfide in the exhaust gas.
One solution researchers have found for the odor problem caused by hydrogen sulfide release is the use of sulfur "getters " such as nickel or iron. These getters hold the sulfur on the catalyst surface (typically as the sulfide) during fuel-rich conditions and release it slowly in a form other than hydrogen sulfide (e.g., SO.sub.2, during lean operation). For example, U.S. Pat. No. 4,780,447 (Kim et al.) discloses a catalyst capable of controlling hydrocarbon, CO, and NO.sub.x emissions as well as hydrogen sulfide emissions. The catalyst contains oxides of Ni and/or Fe as hydrogen sulfide getters as well as noble metals promoted with cerium oxide-rich rare earth oxides. U.S. Pat. No. 4,760,044 (Joy et al.) also discloses the use of an hydrogen sulfide getter, and, in addition, teaches that the getter, in order to be most effective for hydrogen sulfide minimization, must be separated from the primary support (e.g., alumina) and the main catalytic metal (e.g., platinum) to prevent detrimental solid state reactions.
One problem with catalysts employing getters, however, is that getters such as nickel have possible negative environmental and/or carcinogenic effects. One non-nickel containing catalyst which minimizes hydrogen sulfide is disclosed in U.S. Pat. No. 4,868,148 (Henk et al.). The catalyst is a cerium-containing catalyst which minimizes hydrogen sulfide in the emissions by separating the noble metal from the cerium oxide. Another non-nickel cerium-containing catalyst is disclosed in European patent application 0 358 125 (Degussa) where hydrogen sulfide emissions are reduced using a catalyst containing alumina, cerium oxide, boron oxide, and noble metals with zirconia and other oxides as optional components.